Telephone companies generally provide a service for displaying call-related information of a calling party to the called party before the call is answered. The call-related information may include the telephone number and household name of a calling party. One call-related information service, for example, is called Calling Identity Delivery (“Caller ID”). Display of the caller ID information allows a called party the option of not answering an incoming call. In general, basic caller ID information is transmitted from a local telephone company to a called party while the called party's phone is in a hung-up or on-hook state. For call-waiting, caller ID information is displayed even in a pick-up or off-hook state.
Typically, a carrier signal may be modulated in amplitude, phase, or frequency depending upon a particular application. Conventional techniques in the art for modulating the amplitude, phase, and frequency of a carrier signal may include amplitude shift keying (“ASK”), phase shift keying (“PSK”), and frequency shift keying (“FSK”). In any of these modulation techniques, the modulated carrier signal takes on one of two states, that is, either one of two amplitudes, two phases, or two frequencies. The two states of the modulated signal then represent either a logic one or a logic zero.
Conventional specifications in the art for transmitting a carrier signal including caller ID information may use FSK modulation in transmitting data. For example, a 1200-Hz (Hertz) signal denotes a digital logic “one” and a 2200-Hz signal denotes a digital logic “zero.” FSK modulation offers a number of advantages in certain applications over other modulation techniques with respect to noise immunity and average signal power level.
A conventional device 10 for processing FSK signals including caller ID information is shown in FIG. 1. Referring to FIG. 1, device 10 includes a sigma-delta (Σ-Δ or SD) modulator 12, a sinc filter 14, a biquad filter 16, a digital comparator 18, a match filter 20, and a decoder 22. SD modulator 12 receives an input, analog FSK signal including caller ID information and converts the analog FSK signal into a serial digital stream. Sinc filter 14 down-samples the serial digital stream in a digitized sine waveform. The out-band noise of the digitized sine wave is filtered out by biquad filter 16. Digital comparator 18 then shapes the digitized sine wave into a square-wave. Match filter 20 assigns a logic one or logic zero to the digitized FSK signals. Decoder 22 then decodes the output from match filter 20 to display the caller ID information.
The FSK signals are transmitted from an exchanger located in a central office of a telephone service provider, and may be subjected to attenuation during the transmission, resulting in a twist in frequency, baud rate or amplitude of the FSK signals received by device 10. To ensure that a device is able to process attenuated FSK signals including caller ID information, an exchanger issues a test signal to test the device. The test is divided into two parts including:
(1) The exchanger issues an out-band noise signal with signal-to-noise ratio (SNR) of −20 dB at 200 Hz or 3200 Hz; and
(2) The FSK signals include a 1% shift in baud rate, and ±10 dB twist in the amplitude of logic one or logic zero signals.
Biquad filter 16 generally includes an infinite impulse response (“IIR”) filter to filter out out-band noises. However, an IIR produces a non-linear phase output that may adversely affect a FSK signal, particularly when the FSK signal includes an amplitude twist.
FIGS. 2A to 2C are diagrams exemplarily illustrating the distortion of FSK signals caused by biquad filter 16. FIG. 2A shows a waveform of an input FSK signal including a SNR of 20 dB and a twist of +10 dB. As noted above, a 1200-Hz signal denotes a digital logic “one” and a 2200-Hz signal denotes a digital logic “zero”. The +10 dB twist indicates that the amplitude of a 1200-Hz signal is 100.5 times of that of a 2200-Hz signal.
FIG. 2B shows a waveform of a sinc filter output. Referring to FIG. 2B, the 1200-Hz signals are recognizable from the 2200-Hz signals.
FIG. 2C shows a waveform of a biquad filter output. Referring to FIG. 2C, the 1200-Hz signals are hardly recognizable from the 2200-Hz signal due to the phase delay caused by the biquad filter. As a result, it is difficult for a match filter to correctly assign a corresponding logic value to the 1200-Hz and 2200-Hz signals.